The present invention relates generally to the field of acoustic communication and to pipes for gas and oil production, and in particular to transducer arrangements for communicating power and data through metal substrates such as along pipes.
There is growing interest in expanding the capability to measure environmental parameters such as pressure and temperature at any location within the downhole environment present in hydrocarbon wells. These wells often utilize connected steel casing segments to form isolated well sections. Multiple concentric casings of different diameters separated by annular regions may be present in some segments of the well. These casings are used to isolate some regions of the well from other sections such as, for instance, hold back the high hydrostatic pressures present at large depths. Regions may also be separated within a particular string of casing segments using, for instance, a packer which essentially acts as a plug within the casing. Here the packer may separate a high-pressure section of the string from a lower-pressure section.
It is sometimes desirable to place measurement devices to measure environmental parameters such as, for example, temperature and pressure below a packer. The ability to power and communicate with these devices is limited, however, by the need to avoid having any type of electrical or mechanical feed-through such as wires or hydraulic lines pass through the packer, since such a feed-through presents a possible location of weakness that could result in loss of environmental isolation and/or failure. As a result, there is a need for methods to convey power and data from one side of a packer to the other without using physical penetrations of any sort.
Acoustic-electric channels have emerged as a wireless alternative for transmitting data and power through metallic barriers that otherwise block the passage of electromagnetic waves. Most work to date has dealt with the use of a reverberant acoustic-electric channel formed through a single and relatively thin mechanical barrier. Multi-layered acoustic electric channels, however, have the potential to sustain data and power transmission and could pave the way for a wide range of applications in the oil, nuclear and aerospace industries if further developed.
Signal transmission across the length of water filled pipes using ultrasonic waves have been studied previously, for example, by Li Yinghui, S. O. Harrold and L. F. Yeung, “Experimental study on ultrasonic signal transmission within the water-filled pipes,” Mechatronics and Machine Vision in Practice, Proceedings., Fourth Annual Conference on, pp. 93-98, 23-25 Sep. 1997. The effects of multipath was studied and a digital link was setup using a non-coherent detection technique.
An On-Off keying based system was implemented by Bacher et. al (C. Bacher, P. Palensky and S. Mahlknecht, “Low cost data transmission via metallic solids for sensor networking,” Emerging Technologies, Proceedings of the IEEE Symposium on, pp. 193-198, 17-18 Sep. 2005) with an average power consumption of 130 mW to send data across the length of a pipe for a separation of 30 cm using piezoelectric speakers. In another similar application, Lamb waves were used for sending data across the length of an aircraft wing using Frequency Shift Keying (FSK) modulation. H. W. Tomlinson, Jr. and J. B. Deaton, Jr., E. Nieters and F. Ross were awarded the U.S. Pat. No. 7,654,148 for the same on Feb. 2, 2010 (“Ultrasound Communication System for Metal Structure and Related Methods”).
All these systems, however, use narrowband modulation techniques which are ineffective in frequency selective channels as most of the signal energy may get lost in one of the nulls, or attenuative regions, of a given channel.
One approach to wireless data and power transmission through a barrier is taught by R. Primerano in “High Bit-rate Digital Communication through Metal Channels,” PhD dissertation, Drexel University, July 2010, hereafter referred to as Primerano. Primerano is interesting because it teaches Orthogonal Frequency-Division Multiplexing or OFDM modulation with a cyclic prefix to send data at a high rate through a metal wall using ultrasound. The use of OFDM compensates for signal loss due to echoes caused by boundaries or due to other incongruities across the channel. See also Y. Fu, C. Tellambura, and W. A. Krzymien, “Transmitter precoding for ICI reduction in closed-loop MIMO OFDM systems,” Vehicular Technology, IEEE Transactions on, January 2007.